Regarding a method of suppressing howling occurring in an acoustic feedback loop, a variety of howling cancellers have been suggested which insert (allot) a notch filter at a frequency at which the howling is occurring (for example, refer to Patent Document 1). Since the howling may occur at a plurality of frequencies at the same time, it is necessary to insert a plurality of the notch filters having different frequencies. However, the number of the notch filters is limited by performance of hardware configuring the howling canceller. Therefore, when new howling is detected after all notch filters are inserted (i.e., when the number of the notch filters becomes insufficient), the howling canceller should release the notch filter already inserted.
According to a howling removal apparatus disclosed in Patent Document 1, when the number of the notch filters becomes insufficient, the notch filter having the longest insertion time is released and the corresponding notch filter is inserted to suppress the newly detected howling.